Matching state of charge in a string

ABSTRACT

A method of operating a redox flow battery string including at least first and second redox flow batteries and an outside power source includes: providing a least first and second redox flow batteries in a string electrically connected in a string, and each redox flow battery having a state-or-charge (SOC) and an electrical load, wherein the electrical load for at least one of the first and second redox flow batteries in the string is powered by the outside power source; obtaining an SOC value for each redox flow battery in the string; identifying a target SOC value in the string; and adjusting the SOC value for at least one of the first and second redox flow batteries to correspond to the target SOC value by using a portion of stored energy in the at least one first or second redox flow battery to supply power to the electrical load.

BACKGROUND

Concerns over the environmental consequences of burning fossil fuelshave led to an increasing use of renewable energy generated from sourcessuch as solar and wind. The intermittent and varied nature of suchrenewable energy sources, however, has made it difficult to fullyintegrate these energy sources into existing electrical power grids anddistribution networks. A solution to this problem has been to employlarge-scale electrical energy storage (EES) systems. These systems arewidely considered to be an effective approach to improve thereliability, power quality, and economy of renewable energy derived fromsolar or wind sources.

In addition to facilitating the integration of renewable wind and solarenergy, large scale EES systems also may have the potential to provideadditional value to electrical grid management, for example: resourceand market services at the bulk power system level, such as frequencyregulation, spinning reserves, fast ramping capacity, black startcapacity, and alternatives for fossil fuel peaking systems; transmissionand delivery support by increasing capability of existing assets anddeferring grid upgrade investments; micro-grid support; and peak shavingand power shifting.

Among the most promising large-scale EES technologies are redox flowbatteries (RFBs). RFBs are special electrochemical systems that canrepeatedly store and convert megawatt-hours (MWhs) of electrical energyto chemical energy and chemical energy back to electrical energy whenneeded. RFBs are well-suited for energy storage because of their abilityto tolerate fluctuating power supplies, bear repetitive charge/dischargecycles at maximum rates, initiate charge/discharge cycling at any stateof charge, design energy storage capacity and power for a given systemindependently, deliver long cycle life, and operate safely without firehazards inherent in some other designs.

In simplified terms, an RFB electrochemical cell is a device capable ofeither deriving electrical energy from chemical reactions, orfacilitating chemical reactions through the introduction of electricalenergy. In general, an electrochemical cell includes two half-cells,each having an electrolyte. The two half-cells may use the sameelectrolyte, or they may use different electrolytes. With theintroduction of electrical energy, species from one half-cell loseelectrons (oxidation) to their electrode while species from the otherhalf-cell gain electrons (reduction) from their electrode.

Multiple RFB electrochemical cells electrically connected together inseries within a common housing are generally referred to as anelectrochemical “stack”. One or more stacks electrically connected,assembled, and controlled together in a common container are generallyreferred to as a “battery”, and multiple batteries electricallyconnected and controlled together are generally referred to as a“string”. Multiple strings electrically connected and controlledtogether may be generally referred to as a “site”. Sites may beconsidered strings on a larger scale.

A common RFB electrochemical cell configuration includes two opposingelectrodes separated by an ion exchange membrane or other separator, andtwo circulating electrolyte solutions, referred to as the “anolyte” and“catholyte”. The energy conversion between electrical energy andchemical potential occurs instantly at the electrodes when the liquidelectrolyte begins to flow through the cells.

To meet industrial demands for efficient, flexible, rugged, compact, andreliable large-scale ESS systems with rapid, scalable, and low-costdeployment, there is a need for improved RFB systems.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

In accordance with one embodiment of the present disclosure, a method ofoperating a redox flow battery string including at least first andsecond redox flow batteries and an outside power source is provided. Themethod includes: providing a least first and second redox flow batteriesin a string electrically connected in a string, and each redox flowbattery in the string comprising an electrochemical cell in fluidcommunication with anolyte and catholyte storage tanks, and each redoxflow battery having a state-or-charge (SOC) and an electrical load,wherein the electrical load for at least one of the first and secondredox flow batteries is powered by the outside power source; obtainingan SOC value for each redox flow battery in the string; identifying atarget SOC value in the string; and adjusting the SOC value for at leastone of the first and second redox flow batteries in the string tocorrespond to the target SOC value by using a portion of stored energyin at least one first or second redox flow batteries to supply power tothe electrical load.

In accordance with another embodiment of the present disclosure, amethod of operating a redox flow battery string including at least firstand second redox flow batteries and an outside power source is provided.The method includes: providing a least first and second redox flowbatteries in a string electrically connected in a string, and each redoxflow battery in the string comprising an electrochemical cell in fluidcommunication with anolyte and catholyte storage tanks, and each redoxflow battery having a state-or-charge (SOC) and an electrical load,wherein the electrical load is a balance of plant electrical loadrequired to operate each of the plurality of redox flow batteries in thestring, wherein the electrical load for at least one of the first andsecond redox flow batteries is powered by the outside power source;obtaining an SOC value for each redox flow battery in the string;identifying a target SOC value in the string, wherein the target SOCvalue is a function of the SOC values for all of the redox flowbatteries in the string; and adjusting the SOC value for at least one ofthe first and second redox flow batteries in the string to correspond tothe target SOC value by using a portion of stored energy in at least oneof the first or second redox flow batteries to supply power to theelectrical load of at least one of the first and second redox flowbatteries

In any of the embodiments described herein, the method further mayinclude a third redox flow battery in the string.

In any of the embodiments described herein, the electrical load for eachof the redox flow batteries in the string may be a balance of plantelectrical load required to operate each of the plurality of redox flowbatteries in the string.

In any of the embodiments described herein, the target SOC value may bea function of the SOC values for all of the redox flow batteries in thestring.

In any of the embodiments described herein, adjusting the SOC value forat least one redox flow battery in the string to correspond to thetarget SOC value may include reducing the SOC value for at least oneredox flow battery in the string to correspond to the target SOC value.

In any of the embodiments described herein, adjusting the SOC value forat least one of the first and second redox flow batteries in the stringmay be controlled by a battery management system.

In any of the embodiments described herein, each of the redox flowbatteries in the string may be vanadium redox flow batteries, and theSOC value for each redox flow battery in the string may be an opencircuit value (OCV) measurement.

In any of the embodiments described herein, the OCV measurement may bethe difference in electrical potential between selected anolyte andcatholyte reference points for each redox flow battery.

In any of the embodiments described herein, the SOC value for each redoxflow battery in the string may be measured by coulomb counting.

In any of the embodiments described herein, the redox flow batterystring may be an islanded system including at least first and secondredox flow batteries and an outside power source, wherein the outsidepower source is independent of the grid.

In any of the embodiments described herein, the auxiliary power sourcemay be an energy generator or another battery.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is an isometric view of a redox flow battery (RFB) module inaccordance with one embodiment of the present disclosure;

FIG. 2 is an isometric view of the RFB module of FIG. 1 with the outercontainer removed;

FIGS. 3A and 3B are schematic views of various components of the RFBmodule of FIGS. 1 and 2;

FIG. 4 is schematic view of a 1 MW site in accordance with oneembodiment of the present disclosure;

FIG. 5 is a schematic view of a 10 MW site in accordance with oneembodiment of the present disclosure;

FIG. 6 is a control diagram for a site, for example, the sites of FIG. 4or 5;

FIGS. 7-9 are graphical depictions of data regarding capacity managementin an exemplary vanadium RFB string;

FIG. 10 is a schematic view of an RFB module showing an exemplary opencircuit voltage (OCV) measurement;

FIG. 11 is a schematic view of an RFB module showing exemplary balanceof plant (BOP) loads;

FIG. 12 is a schematic view of power delivery to BOP loads in an RFBmodule; and

FIG. 13 is battery system, showing an island system and an optionalconnection to a main grid.

DETAILED DESCRIPTION

Embodiments of the present disclosure are directed to redox flowbatteries (RFBs), systems and components thereof, stacks, strings, andsites, as well as methods of operating the same. Referring to FIGS.1-3B, a redox flow battery 20 in accordance with one embodiment of thepresent disclosure is provided. Multiple redox flow batteries may beconfigured in a “string” of batteries, and multiple strings may beconfigured into a “site” of batteries. Referring to FIG. 4, anon-limiting example of a site is provided, which includes two strings10, each having four RFBs 20. Referring to FIG. 5, another non-limitingexample of a site is provided, which includes twenty strings 10, eachhaving four RFBs 20. RFBs, systems and components thereof, stacks,strings, and sites are described in greater detail below.

Redox Flow Battery

Referring to FIGS. 1 and 2, major components in an RFB 20 include theanolyte and catholyte tank assemblies 22 and 24, the stacks ofelectrochemical cells 30, 32, and 34, a system for circulatingelectrolyte 40, an optional gas management system 94, and a container 50to house all of the components and provide secondary liquid containment.

In the present disclosure, flow electrochemical energy systems aregenerally described in the context of an exemplary vanadium redox flowbattery (VRB), wherein a V³⁺/V²⁺ sulfate solution serves as the negativeelectrolyte (“anolyte”) and a V⁵⁺/V⁴⁺ sulfate solution serves as thepositive electrolyte (“catholyte”). However, other redox chemistries arecontemplated and within the scope of the present disclosure, including,as non-limiting examples, V²⁺/V³⁺ vs. Br⁻/ClBr₂, Br₂/Br⁻ vs. S/S²⁻,Br⁻/Br₂ vs. Zn²⁺/Zn, Ce⁴⁺/Ce³⁺ vs. V²⁺/V³⁺, Fe³⁺/Fe²⁺ vs. Br₂/Br⁻,Mn²⁺/Mn³⁺ vs. Br₂/Br⁻, Fe³⁺/Fe²⁺ vs. Ti²⁺/Ti⁴⁺, etc.

As a non-limiting example, in a vanadium flow redox battery (VRB) priorto charging, the initial anolyte solution and catholyte solution eachinclude identical concentrations of V³⁺ and V⁴⁺. Upon charge, thevanadium ions in the anolyte solution are reduced to V²⁺/V³⁺ while thevanadium ions in the catholyte solution are oxidized to V⁴⁺/V⁵⁺.

Referring to the schematic in FIG. 3A, general operation of the redoxflow battery system 20 of FIGS. 1 and 2 will be described. The redoxflow battery system 20 operates by circulating the anolyte and thecatholyte from their respective tanks that are part of the tankassemblies 22 and 24 into the electrochemical cells, e.g., 30 and 32.(Although only two electrochemical cells are needed to form a stack ofcells, additional electrochemical cells in the illustrated embodiment ofFIG. 3A include electrochemical cells 31, 33 and 35.) The cells 30 and32 operate to discharge or store energy as directed by power and controlelements in electrical communication with the electrochemical cells 30and 32.

In one mode (sometimes referred to as the “charging” mode), power andcontrol elements connected to a power source operate to store electricalenergy as chemical potential in the catholyte and anolyte. The powersource can be any power source known to generate electrical power,including renewable power sources, such as wind, solar, andhydroelectric. Traditional power sources, such as combustion, can alsobe used.

In a second (“discharge”) mode of operation, the redox flow batterysystem 20 is operated to transform chemical potential stored in thecatholyte and anolyte into electrical energy that is then discharged ondemand by power and control elements that supply an electrical load.

Each electrochemical cell 30 in the system 20 includes a positiveelectrode, a negative electrode, at least one catholyte channel, atleast one anolyte channel, and an ion transfer membrane separating thecatholyte channel and the anolyte channel. The ion transfer membraneseparates the electrochemical cell into a positive side and a negativeside. Selected ions (e.g., H+) are allowed to transport across an iontransfer membrane as part of the electrochemical charge and dischargeprocess. The positive and negative electrodes are configured to causeelectrons to flow along an axis normal to the ion transfer membraneduring electrochemical cell charge and discharge (see, e.g., line 52 inFIG. 3A). As can be seen in FIG. 3A, fluid inlets 48 and 44 and outlets46 and 42 are configured to allow integration of the electrochemicalcells 30 and 32 into the redox flow battery system 20.

To obtain high voltage, high power systems, a plurality of singleelectrochemical cells may be assembled together in series to form astack of electrochemical cells (referred to herein as a “stack,” a “cellstack,” or an “electrochemical cell stack”), e.g., 30 or 32 in FIG. 3A.Several cell stacks may then be further assembled together to form abattery system 20. Stacks may be connected in strings in series or inparallel. A MW-level RFB system generally has a plurality of cellstacks, for example, with each cell stack having more than twentyelectrochemical cells. As described for individual electrochemicalcells, the stack is also arranged with positive and negative currentcollectors that cause electrons to flow through the cell stack generallyalong an axis normal to the ion transfer membranes and currentcollectors during electrochemical charge and discharge (see, e.g., line52 shown in FIG. 3A).

At any given time during battery system 20 charging or discharging mode,reactions only occur for the electrolyte that is contained insideelectrochemical cells. The energy stored in the battery system 20increases or decreases according to the charging and discharging powerapplied to the electrochemical cells.

String and Site Control System

As noted above, a string 10 is a building block for a multiple MW site.As seen in the exemplary layouts in FIGS. 4 and 5, each string 10includes four battery containers connected in series to a power andcontrol system (PCS) 12 container. As can be seen in FIG. 6, the controlsystem for each string includes a battery management system (BMS) 14with local control provided, for example, by a human machine interface(HMI). The BMS 14 interprets remote commands from the site controller18, for example, a customer requirement to charge or discharge, as itsimultaneously directs the appropriate operations for each battery andsub-component in the string 10 via a communication network. At the sametime, according to programmed logic, the BMS 14 interprets string 10operating data from the batteries 20, PCS, and their associatedsub-components to evaluate service or diagnose maintenance requirements.See also FIG. 6 for string and site control diagrams.

As a non-limiting example, an exemplary VRB may have capacity up to 125kW for four hours (500 kW-hours) and a storage string may have capacityup to 500 kW for four hours (2 MW-hours). To be effective as a largescale energy storage system that can be operated to provide multiplelayered value streams, individual batteries, designed and manufacturedto meet economies of scale, may be assembled as building blocks to formmultiple-megawatt sites, for example 5 MW, 10 MW, 20 MW, 50 MW, or more.Managing these large installations requires multi-level control systems,performance monitoring, and implementation of various communicationsprotocols.

Referring to FIG. 4, an exemplary 1 MW system layout shows two 500 kWbuilding block sub-assemblies or strings 10 that each include fourbattery modules 20 and one PCS module 102. Using this approach,multi-level larger systems may be assembled, for example, thesingle-level 10 MW system shown in FIG. 5. As described in greaterdetail below, the unique combination of systems and components describedherein provide significantly more energy density in a compact flowingelectrolyte battery module 20 and string 10 design than previouslydesigned flowing electrolyte batteries, such earlier generation VRBs.Other hybrid flowing electrolyte batteries, such as ZnBr2 systems, maydemonstrate similar characteristics.

Battery Container System, Electrolyte Tank Assembly, and GeneralArrangement

Referring now to FIGS. 1 and 2, each RFB 20 includes a container 50 thathouses the remaining components of the system in a substantially closedmanner. These remaining components generally include the anolyte andcatholyte tank assemblies 22 and 24, the stacks of electrochemical cells30, 32, and 34, a system for circulating electrolyte 40, and an optionala gas management system 94. The configuration of each of thesecomponents will now be described in more detail.

FIG. 1 depicts the container 50 that houses, for example, the componentsshown in FIG. 2. The container 50 can be configured in some embodimentsto be an integrated structure that facilitates or provides one or moreof the following characteristics: compact design, ease of assembly,transportability, compact multiple-container arrangements andstructures, accessibility for maintenance, and secondary containment.

In the illustrated embodiment of FIGS. 1 and 2, the representativecontainer 50 comprises two major compartments that house components ofthe RFB 20. In some embodiments, the division between the first andsecond compartments 60 and 62 is a physical barrier in the form of abulkhead 70 (see FIG. 3B), which may be a structural or non-structuraldivider. The bulkhead 70 in some embodiments can be configured toprovide secondary containment of the electrolyte stored in tankassemblies 22 and 24. In another embodiment, a secondary structural ornon-structural division can be employed to provide a physical barrierbetween the anolyte tank 22 and the catholyte tank 24. In either case,as will be described in more detail below, the tanks 22 and 24 areconfigured as so to be closely fitted within the compartment orcompartments, thereby maximizing the storage volume of electrolytewithin the container 50, which is directly proportional to the energystorage of the battery 20.

In some embodiments, the container 50 has a standard dimensioning of a20 foot ISO shipping container. In one representative embodiment shownin FIGS. 1 and 2, the container has a length A which may be 20 feet, 8feet in width B, and 9½ feet in height C, sometimes referred to as aHigh-Cube ISO shipping container. Other embodiments may employ ISOdimensioned shipping containers having either 8 feet or 8½ feet inheight C, and in some embodiments, up to 53 feet in length A. In some ofthese embodiments, the container 50 can be additionally configured tomeet ISO shipping container certification standards for registration andease of transportation via rail, cargo ship, or other possible shippingchannels. In other embodiments, the container may be similarlyconfigured like an ISO shipping container. In other embodiments, thecontainer has a length in the range of 10-53 feet and a height in therange of 7-10 feet.

The container 50 also includes various features to allow for the RFB 20to be easily placed in service and maintained on site. For example,pass-through fittings are provided for passage of electrical cablingthat transfers the power generated from circulation of the anolyte andthe catholyte through the stacks of electrochemical cells. In someembodiments, the container 50 includes an access hatch 80, as shown inFIG. 1. Other hatches, doors, etc. (not shown) may be included forproviding access to systems of the RFB 20.

String Capacity Management of Electrolyte

Passive capacity management techniques have been shown to maintainstable performance under most conditions for a single battery. However,other operating conditions may occur that require active capacitymanagement, especially on the string and site level.

Described herein are systems and methods of operation designed forimproving performance on a string and site level. For example, in someembodiments of the present disclosure, performance can be improved bymatching the state of charge when a string includes multiple batterieshaving different states of charge. In other embodiments of the presentdisclosure, when an islanded system is turned off, stored energy can bepreserved and used to restart the system on its own.

In one example, stack variation caused by differences in manufacturingassembly and materials may produce slightly different performancecharacteristics between each of the four RFBs 20 in a string 10 (seeexemplary string diagrams in FIGS. 2 and 6), in some cases leading todifferent membrane ion transfer capabilities or different levels of sidereactions, both of which contribute to performance mismatch in a stringof batteries. One mechanism that may be affected by manufacturingdifferences in stacks can be seen during battery operation in the wayions travel back and forth through the membrane separating positive andnegative electrolytes as they form a closed electrical circuit, and inthe way water molecules travel through the membrane together with otherhydrated ions or by themselves. As a result of stack differences, thevolume of the positive and negative electrolytes and the concentrationsof active ions in the electrolytes may change at different rates duringbattery operation.

In another example, stack variations caused by damage (leakage,blockage, etc.) to one or more stack cells may produce slightlydifferent performance characteristics when the stacks are assembled asbatteries and strings, and may also cause an imbalance in thepredetermined battery tank volume ratio described above. Other reasonsfor stack variation may include differences in the electrode, stackcompression, etc.

Because there may be performance differences between batteries in astring and all batteries in a string are electrically connected forcharge and discharge operations, the worst performing battery maydetermine the performance of the string. Further, because each batteryin the string has dedicated electrolyte tanks, lower performingbatteries may continue to experience declining performance caused, forexample by the by stack variation described above. Declining batterycapacity is generally indicative of or may lead to electrolyte stabilityand capacity problems for the associated string. If left unchecked,these performance variations may result in decreased capacity across astring (or a site).

The possible effect of decreasing performance of one or more batteriesin a string is illustrated below in EXAMPLES 1 and 2, using data basedon open circuit voltage (OCV) values measured on the cell, stack, andbattery level for each RFB in a string. OCV directly corresponds to SOCand is one measure of the state-of-charge (SOC) of a vanadium redox flowbattery (VRFB), and is defined as the difference in electrical potentialbetween two terminals of a device when it is disconnected from thecircuit, for example, selected anolyte and catholyte reference pointsfor each redox flow battery (see, e.g., FIG. 10).

Matching SOC in a string mitigates performance degradation of a batterystring, as illustrated below in EXAMPLE 3.

Example 1 Energy Density

In a string of three, series-connected, kW-scale batteries withoutcapacity management adjustments, a steady decline in energy density over35 cycles can be seen in FIG. 7.

Example 2 Open Circuit Voltage

In a string of three, series-connected, kW-scale batteries withoutcapacity management adjustments, a steady deviation in open circuitvoltage (OCV) at the end of discharge over 35 cycles can be seen in FIG.8.

Example 3 Stack Performance Recovery

In a string of three, series-connected, kW-scale batteries with capacitymanagement adjustments, the energy density decline of about 7% is shownin FIG. 9 for over 200 cycles. As compared to the energy density declinein FIG. 7 of about 7% over only 35 cycles, matching operation mitigatesperformance degradation effects in one or more batteries in a string.

Active Electrolyte State-of-Charge (SOC) Measurements

To manage battery capacity on the string (or site) level,state-of-charge (SOC) values are determined for each RFB. See FIG. 6showing “Battery OCV” determinations for each battery in the string. Asone non-limiting example in a vanadium redox flow battery (VRFB), SOCcan be determined using an open circuit voltage (OCV) measurement, whichis the difference in electrical potential between two terminals of adevice when it is disconnected from the circuit. For example, as shownin FIG. 10, OCV for a VRFB can be measured using a small electrochemicalcell as the potential between the anolyte and catholyte solutions.

Other ways of determining SOC besides OCV are also within the scope ofthe present disclosure, such as recording and analyzing the amount ofenergy entering and leaving the battery over a given time period, whichmay be referred to as coulomb counting.

Determining Target State-of-Charge (SOC) Value

After determining SOC, a selected SOC value can be determined as atarget value for the other batteries in the system. Therefore, inaccordance with one embodiment of the present disclosure, the other RFBsin the string can then be adjusted to correspond to the selected SOCvalue. The target SOC value is a function of the SOC values for all ofthe plurality of redox flow batteries in the string.

As a non-limiting example, the target SOC value may be the lowest SOCvalue in the string.

As another non-limiting example, the target SOC value may be an averagestring SOC, which may or may not omit the underperforming battery fromthe calculations.

As another non-limiting example, the target SOC value may be a maximumdeviation from the average string SOC value.

As another non-limiting example, the target SOC value may be a targetSOC value determined by an algorithm based on conditions in the string.The target SOC value may be a conditional value based on transientoperating parameters.

In a dynamic system, the predetermined or target value will changecontinually based on changing conditions in the string. The target SOCvalue may be subject to change based on one or more of the followingconditions: low SOC; an unusually large load on the system; highdischarge; and other external conditions.

Adjusting the target SOC may be controlled by the battery managementsystem (BMS) during battery operation or may be performed duringmaintenance of the redox flow battery.

Adjusting SOC to Match State-of-Charge in a Battery String

In accordance with embodiments of the present disclosure, a method ofoperating a redox flow battery string is provided. The string includes aplurality of redox flow batteries, for example, at least first andsecond redox flow batteries. The string also includes an outside powersource that provides power to operate the string.

The outside power source may be a main power source, such as a grid, ormay be a secondary power source, such as a non-grid power source, forexample, a generator or other auxiliary power device.

The plurality of redox flow batteries in the string are electricallyconnected in series or parallel.

In accordance with embodiments of the present disclosure, one exemplarymethod for adjusting the SOC value for at least one of the redox flowbatteries in the string to correspond to the target SOC value includesusing a portion of the stored energy in the at least one redox flowbattery to supply power to the electrical load that operates the atleast one redox flow battery. In one embodiment of the presentdisclosure, adjusting the SOC value for at least one redox flow batteryin the string to correspond to the target SOC value includes reducingthe SOC value for at least one redox flow battery in the string tocorrespond to the target SOC value.

The electrical load, also referred to as the balance-of-plant (BOP)load, for the exemplary 125 kW redox flow battery shown in FIG. 4 isapproximately 3 kW. Referring to FIGS. 6 and 11, the exemplary BOP loadcomprises auxiliary power required for operating battery componentequipment such as the anolyte and catholyte electrolyte pumps 90,cooling system fans 92, instrumentation and electrical control systems96, battery management system 14, etc. “BOP loads” 100 are shown foreach redox flow battery in the string.

Supplying power to the electrical load that operates the at least oneredox flow battery may include a portion of BOP load or all of the BOPloads for the RFB. For example, different components of the RFB system20 (for example, those shown in FIG. 11) may be turned off or on toreduce or increase the BOP load.

Referring to FIG. 12, the battery system has a dual feed control powersystem. In that regard, BOP loads 100 may be powered using an externalpower source 102 or an internal power source 104 derived from the energystored in the redox flow battery system 20. In the illustratedembodiment of FIG. 12, power is only supplied from either an externalpower source 102 or an internal power source 104. However, a combinationof power sources is within the scope of the present disclosure.

In the illustrated embodiment of FIG. 12, power is supplied to the BOPloads using a 24 VDC bus 108. Diodes 106 in the illustrated embodimentof FIG. 14 prevent the reverse flow of power.

Typically, the BOP load 100 is powered by an outside power source 102.When the SOC of a battery in a string is high relative to the otherbatteries, the stored energy in the battery from the internal powersource 102 can be used to power its BOP load 100 to reduce the SOC ofthe battery. Therefore, at least one battery 20 in the string 10 usesstored energy in the battery to power its BOP load 100 and at leastanother battery in the string uses energy from an outside power sourceto power its BOP load.

Adjustment to reduce the SOC value of the battery having a high SOCvalue such that its SOC value is closer to a target value to provide acloser match of the SOC value(s) of the other batteries in the string.Such matching operation helps to mitigate performance degradation of abattery string. In addition, matching can help to compensate formanufacturing tolerances in battery module performance.

Islanded Power Systems

A main power grid is an interconnected network for delivering electricalpower, typically produced by large-scale power stations. An islandedpower system is a power system that operates, or is capable ofoperating, independent of a main power grid.

A micro-grid is one example of an islanded power system. A micro-grid isan islanded power system that is much smaller in scale than the mainpower grid. A micro-grid may be used, for example, to power remotefacilities or communities where connections to the main power grid areunavailable, or to provide backup power in the event of an outage in themain power grid. A micro-grid may be coupled to the main power grid. Amicro-grid may include a switch that allows the micro-grid to operate ina grid-connected mode or in an island state to operate independent ofthe main power grid. Island state can be useful, for example, when themain power grid (or a connection to the main power grid) is notfunctioning or is unreliable. Alternatively, a micro-grid may operatealways in an island state, such as in remote locations where connectionsto the main power grid are not available.

FIG. 13 is a block diagram of an illustrative micro-grid 170. In theexample shown in FIG. 13, the micro-grid 170 includes a battery system110 comprising a control circuit 114 (e.g., a BMS as described above)and one or more batteries 120 (e.g., a string of RFBs as describedabove). As shown, the battery system 110 is connected to one or moreloads 150 (e.g., lighting systems, heating/cooling systems, or otherelectrical loads) on the micro-grid that can be powered by the batterysystem. The micro-grid 170 also may include other power sources such asone or more generators 140 (e.g., diesel generators, wind-poweredgenerators) or other power sources (e.g., solar panel arrays), and/orone or more other energy storage devices 160 separate from the batterysystem 110, and/or one or more optional connected assets 130.

The micro-grid 170 also includes an optional connection 180 to a mainpower grid. If the optional connection to the main power grid ispresent, the micro-grid also includes a switch (not shown) to allow themicro-grid to operate in a grid-connected mode or in an island state.

Using System Stored Energy for Black Start

In some situations, a battery system 20 in accordance with embodimentsof the present disclosure may be turned off with the battery system 20having a stored amount of energy. To turn back on, the battery system 20may rely on energy from an outside energy source, such as a main powergrid or an auxiliary power source, to power start up functions in thebattery system 20. When the battery system 20 is in an island state, itis not connected to the main power grid, and may not have auxiliarypower resources for start-up.

In accordance with embodiments of the present disclosure, when a batterysystem is in an “island” state and an “off” state, the battery system 20includes a method for preserving power reserves to restart andrestarting by using stored power in the battery system 20. Referring toFIG. 3A, power is stored in the electrolyte 40 in the electrochemicalcells 30, 32, 34 of each redox flow battery 20 after a batterytransitions from an “on” state to an “off” state.

When islanded, the system may be configured to be in an island state, ormay be forced into an island state as a result of the loss of gridpower, for example, in the event of a grid failure.

An amount of stored energy in the electrochemical cell when the redoxflow battery is in the off state can be maintained by exchangingelectrolyte from the anolyte and catholyte storage tanks with theelectrolyte in the electrochemical cell. The electrolyte containingstored energy in the electrochemical cell has a state of charge (SOC)value, which may dissipate and lose capacity over time as a result ofself-discharge.

Therefore, to counteract lost energy storage capacity and maintain abaseline SOC in the electrochemical cell, stored energy remaining in theelectrolyte in the electrochemical cell can be used to transfer newelectrolyte with higher energy capacity from the anolyte and catholytestorage tanks into the electrochemical cell. The reduced energy capacityelectrolyte, in exchange, is returned to the anolyte and catholytestorage tanks.

Such exchange may occur on a periodic basis, for example, continuously,as a result of demand from the system, as a result of a SOCdetermination, or based on a predetermined time schedule. Asnon-limiting examples, predetermined time schedules may be once every 10hours, once every 20 hours, once every 40 hours, or any other timeschedule based on known information about the system, whether regular orirregular.

As a non-limiting example, in the exemplary 125 kW system shown in FIG.4, the maximum amount of energy stored in the electrochemical cells ofthe stacks that is available for black start at full SOC isapproximately 8 kWh. When the system is caused to enter an off state inthis condition, the energy stored in the electrochemical cells of thestacks and available to maintain black start ready state decreases byapproximately 2% per hour. After 20 hours, the remaining energy isapproximately 4.8 kWh. After 40 hours, the approximately 1.6 kWh ofenergy available in the electrochemical cells of the stacks should beused to power the system to replenish the electrochemical cells withcharged electrolyte from the storage tanks to ensure continued blackstart availability.

The transfer of electrolyte may require stored energy to run transferpumps and/or other control systems in the redox flow battery,collectively referred to above as BOP. The transfer pumps may include amain transfer pump for the overall system, auxiliary pumps, orindividual catholyte and anolyte transfer pumps. Other components whichmay require stored energy include instrumentation, for example, for SOCdetermination and the battery management system (BMS) which controlsbattery operation.

When the battery is ready to be restarted, the stored energy remainingin the electrolyte in the electrochemical cell can be used to restartthe battery to its on state.

A periodic state of charge (SOC) determination can identify the amountof stored energy in the electrolytic cells when the system is turned offand the electrolyte pumps are stopped.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the disclosure.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method of operating aredox flow battery string including at least first and second redox flowbatteries and an outside power source, comprising: providing a leastfirst and second redox flow batteries in a string electrically connectedin a string, and each redox flow battery in the string comprising anelectrochemical cell in fluid communication with anolyte and catholytestorage tanks, and each redox flow battery having a state-or-charge(SOC) and an electrical load, wherein the electrical load for at leastone of the first and second redox flow batteries is powered by theoutside power source; obtaining an SOC value for each redox flow batteryin the string; identifying a target SOC value in the string; andadjusting the SOC value for at least one of the first and second redoxflow batteries in the string to correspond to the target SOC value byusing a portion of stored energy in at least one first or second redoxflow batteries to supply power to the electrical load.
 2. The method ofclaim 1, further comprising a third redox flow battery in the string. 3.The method of claim 1, wherein the electrical load for each of the redoxflow batteries in the string is a balance of plant electrical loadrequired to operate each of the plurality of redox flow batteries in thestring.
 4. The method of claim 1, wherein the target SOC value is afunction of the SOC values for all of the redox flow batteries in thestring.
 5. The method of claim 1, wherein adjusting the SOC value for atleast one redox flow battery in the string to correspond to the targetSOC value includes reducing the SOC value for at least one redox flowbattery in the string to correspond to the target SOC value.
 6. Themethod of claim 1, wherein adjusting the SOC value for at least one ofthe first and second redox flow batteries in the string is controlled bya battery management system.
 7. The method of claim 1, wherein each ofthe redox flow batteries in the string are vanadium redox flowbatteries, and wherein the SOC value for each redox flow battery in thestring is an open circuit value (OCV) measurement.
 8. The method ofclaim 7, wherein the OCV measurement is the difference in electricalpotential between selected anolyte and catholyte reference points foreach redox flow battery.
 9. The method of claim 1, wherein the SOC valuefor each redox flow battery in the string is measured by coulombcounting.
 10. The method of claim 1, wherein the redox flow batterystring is an islanded system including at least first and second redoxflow batteries and an outside power source, wherein the outside powersource is independent of the grid.
 11. The method of claim 10, whereinthe auxiliary power source is an energy generator or another battery.12. A method of operating a redox flow battery string including at leastfirst and second redox flow batteries and an outside power source,comprising: providing a least first and second redox flow batteries in astring electrically connected in a string, and each redox flow batteryin the string comprising an electrochemical cell in fluid communicationwith anolyte and catholyte storage tanks, and each redox flow batteryhaving a state-or-charge (SOC) and an electrical load, wherein theelectrical load is a balance of plant electrical load required tooperate each of the plurality of redox flow batteries in the string,wherein the electrical load for at least one of the first and secondredox flow batteries is powered by the outside power source; obtainingan SOC value for each redox flow battery in the string; identifying atarget SOC value in the string, wherein the target SOC value is afunction of the SOC values for all of the redox flow batteries in thestring; and adjusting the SOC value for at least one of the first andsecond redox flow batteries in the string to correspond to the targetSOC value by using a portion of stored energy in at least one of thefirst or second redox flow batteries to supply power to the electricalload of at least one of the first and second redox flow batteries.